Both clinical and experimental data have shown that the cerebral hemispheres in man are not simply functional duplicates of each other, but are specialized to process information in different ways. Recently, controversy has emerged as to whether this functional specialization is present at birth, or whether it develops gradually during the early years of life. Much of the early evidence pertaining to cerebral specialization came from clinical behavioral studies on patients with lesions. A recent trend, however, has been to use electrophysiological methods (e.g., evoked potentials, EEG) because they allow study of the intact human while more directly measuring the underlying physiology of cerebral specialization. Although work with these methods has not been uniformly successful, recent work in our laboratory with adults has led to the development of a paradigm using auditory evoked potentials in which lateralized brain functioning has been reliably detected. The proposed investigations aim to extend this and other evoked potential techniques to study the development of cerebral functional specialization as it largely pertains to language reception and perception of music and thus provide data about the development of lateralization. Two major paradigms are proposed to study cerebral functional specialization in infants and children. One paradigm, proven successful with adults in our laboratory, involves the presentation of auditory tone pips while subjects are engaged in tasks designed to activate specific hemispheric sites. The second relates auditory evoked potentials to the acquisition of meaning in infants. It is expected that words will elicit different evoked potentials as a function of the acquisition of their meaning by the child. Both longitudinal and cross-sectional designs are incorporated into this proposal, and sex differences will enter as a factor in all analyses. The proposed studies should provide useful basic information about the central nervous system substrates of language development and cerebral functional differences in infancy and early childhood.